Our planet is surrounded by air, and this air has mass. So what is air pressure?
The mass of air at 1g force (standard gravity) creates an ambient atmospheric pressure of approximately 1bar (1000 millibar) at sea level.
A typical weather map such as this one shows isobars which connect points of equal atmospheric pressure.
High pressure fields result in good weather, whereas low-pressure systems bring rain and storms.
Today, air pressure is normally indicated in millibars (mbar) or Pascals (Pa):
100Pa = 1mbar
In earlier times, the units used were mmHg (millimetres of mercury):
1mmHg = 1.3mbar